The present invention relates to the process control of a heterogeneous production line, for example, of semiconductor manufacturing. More especially, the present invention relates to a mechanism for monitoring the real-time operation performance of a heterogeneous production line.
In a semiconductor fabrication factory, there are sometimes various productions and various processes managed and controlled in a production line at the same time, especially for those productions to order. Short cycle times and precise delivery schedules are always pursued as major goals and tasks for the purpose to satisfy the expectations of customer. For attaining a greater throughput and providing completion of the lot before the due date (DD) with short cycle time, complex sets of product mix and process mix are usually arranged to optimize delivery efficiency. In a heterogeneous production arrangement as mentioned above, various products processed simultaneously in a production line are often dispatched to a test process.
In such a heterogeneous production line with mixed-product operation, different products will be processed over different process stages with different stage time even in the same stage. In order for foundry supervisors to dispatch lots more effectively and provide a better operation arrangement of product and process mixes, a foundry fabrication operation must be sketched with some characteristics that can present the information needed.
In the past, work in process (WIP) is often listed to display the wafer numbers of those lots in every stage of a production line, and a clock time is usually adopted to indicate the actual amount of time that has already been consumed for the lot in a stage. However, the information that can be provided by WIP and the clock time is limited. Supervisors must utilize this WIP and clock time with their experience to get further information about the conditions of the productions. Sometimes, it could even provide a misleading message for reporting the operation efficiency. The same clock time represents the different meaning for productions having different cycle time. For example, a same clock time of about two hours may be not enough for a process requiring three-hour cycle time, but will be too much for a process only requiring one-hour cycle time.
FIG. 1 displays a stage WIP report of a four-stage production line, wherein every row represents the WIPs of a lot, and every column represents the WIPs of a stage. In this report, only the wafer numbers are shown and no other status such as clock times can be found. FIG. 2 displays a stage clock time report of the same production line. The amounts of the clock time of all lots at all stages are shown, but the clock time tells not much without comparison to the ideal cycle time, because it neglect the variance between the ideal cycle time needed for different products. It needs an experienced supervisor to concentrate himself to comprehend what a clock time list, such as the one shown in FIG. 2, tells.
Even an experienced supervisor can not be aware from the WIP and clock time reports that, does the current process fall behind the scheduled progress in consideration of all the processes as a whole rather than the individual process. There is a possibility that, although the current process achieves percentage of scheduled progress, the remaining time is inadequate for completing all the subsequent processes before the due date. It could also be possible that although the current process falls behind the scheduled progress, the remaining time is still ample for completing all the subsequent processes before the due date.
Hence, there is no sufficient information can be found in the present monitoring mechanism for the supervisor to judge in the real time that, is the remaining time till the due date sufficient, or insufficient, for the subsequent processes. Most of the issues arising in the processes will not come into awareness until post analysis, which can not help to reduce the cycle time while in process. A real-time monitoring mechanism is still lacked, or at least not good enough, in the present system.
The present invention proposes a method for monitoring the real-time production operation. This method utilizes critical ratios for characterizing the production statuses and indicating the critical degrees of lots at different stages. Color codes are applied to denote the critical degrees.
The used stage time of a lot already used at a stage, or the used waiting time already used to waiting in the stage, and the theoretical remaining processing time anticipated to be used for all the remaining processes, is counted. The allowed stage time anticipated for the lot to be expended at the stage, or the allowed waiting time allowed to wait at the stage, and the allowed slack time from the present until the target-out time, is estimated. The target-out time is the time scheduled to complete all the processes for the lot. The critical stage ratio, or the critical waiting ratio, and the critical slack ratio are calculated by the following equations:
critical stage ratio=allowed stage time/used stage time,
critical slack ratio=allowed slack time/theoretical remaining processing time,
critical waiting ratio=allowed waiting time/used waiting time.
Thereafter, the status of the lot in a stage is graded according to its critical stage ratio, critical slack ratio, and critical waiting ratio. Color codes are used to denote the critical degrees for different ranges of critical ratios. A stage critical degree report of a heterogeneous production line is tabled to display all the statuses of the stage in this production line. The WIPs, color codes of the critical stage ratio, the critical slack ratio and the critical waiting ratio are all displayed.